1. Field of the Invention
The present invention relates to a distribution band controlling method for a packet including video or audio data, a packet distributing apparatus, and a video distributing system.
2. Description of the Related Art
The demand for a data communication such as an Internet communication, etc. of image data, audio data, etc. via various communication means has been increasing. As communication services using the Internet, video-on-demand, the streaming distribution of a live video, a teleconference system, etc. exist.
With the stream distribution, image data must be transmitted in real time, and RTP (Real-time Transport Protocol) is known as an Internet technique suitable for such a real-time communication. With a data communication using RTP, a transmitting side adds a time stamp to a packet as time information, and a receiving side reproduces a received packet in order of time by referencing the time stamp. As a result, synchronous reproduction can be made without being influenced by a delay or a jitter at the time of a packet transfer.
However, since RTP does not guarantee a real-time data communication, a distribution delay or a packet loss can occur on a network for a packet based on RTP similar to packets of other protocols. Therefore, there is a problem that a receiving side cannot make reproduction while maintaining quality even if a data distribution process of the high quality is executed.
To overcome the above described problem in a data transfer using RTP, a packet retransmission request, and the transmission of a retransmission packet are considered to be made by using TCP (Transmission Control Protocol) having high reliability of a data transfer.
However, the throughput and the delay of TCP are respectively low and large although it is good at an error correction. Accordingly, there is a possibility that a packet does not reach in time for reproduction even if it is retransmitted in response to a retransmission request on a decoder side, and TCP has a problem from the viewpoint of the implementation of a real-time communication.
As an error correction method when a packet loss occurs, an FEC (Forward Error Correction) technique is known.
FIG. 10 explains an error correction using FEC in a video distributing system.
In FIG. 10, an encoder device 101 generates a redundant parity P1 from transmission data D1 and D2 in an EFC encoding processing unit 102, and distributes data packets storing the transmission data D1, D2, . . . , and a redundant packet storing the redundant parity to a decoder device 103.
If a packet loss occurs on the side of the decoder device 103, an FEC restoration processing unit 104 restores the lost packet by using the redundant packet. For example, if a packet storing the data D1 is lost, the data D1 is restored from the redundant packet P1 and the data D2.
With the above described error correction using FEC, it becomes difficult to restore a packet if a bursty packet loss occurs.
As an error correction method used when a bursty packet loss occurs, a technique called ARQ (Automatic Repeat Request) is known. FIG. 11 explains this technique.
This error correction method compensates for a packet loss in a way such that a decoder device 112 transmits a request to retransmit a lost packet to an encoder device 111 the moment when the packet loss occurs, and the encoder device 111 distributes the requested packet to the decoder device 112.
Patent Document 1 recites that an error control of FEC or ARQ is dynamically changed based on the state of a network monitored by a network state monitoring unit.
However, the error control based on the state of a network, which is recited by Patent Document 1, only means that an error correction using ARQ is made if a both-way propagation delay (RTT) of the network is shorter as a result of determining whether the both-way propagation delay is either longer or shorter than a predetermined value, and an error correction using FEC is made if the RTT is longer.    Patent Document 1: Japanese Published Unexamined Patent Application No. 2003-179580